This invention generally relates to a polling technique for a plurality of devices connected together by a two-wire loop. More specifically, it relates to a global polling technique for a plurality of devices in a security system in which the devices are coupled together by a two-wire loop.
Security systems may comprise a plurality of devices which are coupled together via a wire link. In simple security systems, these devices may be magnetic or mechanical switches or other well known devices which either open or close a connection on the line. However, in more sophisticated systems, each point of a multi-point security system is assigned an address and communicates its response to a central controller along with the address so that the location of the intrusion in known. Such security systems may employ lengthy loops of wire to couple the devices to the central controller, with a length of several thousand feet being a typical maximum for each loop. Accordingly, minimalization of the number of wires in the loop is desirable in order to minimize the wire costs associated with the wire loops.
Serially addressable receiver/transmitter circuits are known which can transmit and receive data between points in the loop and a central controller along the same lines which are also utilized to transmit power. The devices typically utilize a three-level line signal, as shown as signal 100 in FIG. 1. The signal typically comprises a high voltage level, a reference voltage level, and a third logic level in between the first two levels. The circuit rectifies and filters the high level on the line in order to obtain power for the circuitry. The devices may be typically embodied in a single integrated circuit such as model CS-212 manufactured by Cherry Semiconductor. The devices known in the art utilize a unit poll. That is, an address is sent via a central controller along the wire loop. This is contiguously followed by a series of bit sequences corresponding to logic ones, for example, having a length equal to the number of bits in a message expected to be received from one of the serially addressed devices. The serially addressed device in the loop having the corresponding address is activated by the address on the line and responds by modifying the bit sequences that follow the address so as to transmit a message to the controller. If the bit in the bit sequence that is being transmitted by the controller is desired to be a logic one, for example, the serially addressable device would not change the bit and thus a one would be transmitted to the controller. If it was desired that the bit actually represent a zero, the serially addressable device would place a low impedance across the line in order to force the voltage on the line to a zero, thus transmitting a zero to the controller.
In loops operating on this principle, it is necessary that the controller periodically sequence through all of the addresses of the devices on the loop in order to obtain new messages from each of the devices. In a multi-point security system, the data on the loop can represent an intrusion, an ARMING or DISARMING of the system, a failure within the system or the need to change a battery at one of the points, for example. It is thus necessary that the polling take place so that each device is addressed within a relatively short interval, so that these actions can take place in real time. As the number of devices in the loop increases, it is also necessary to increase the number of bits in the address. Furthermore, an addressing scheme that can be utilized employs an Applications Specific integrated circuit (ASIC) that, in addition to the function that it performs in the security system, is programmed with a unique serial number. The type of functions performed by the ASIC, such as a security detector data receiver/transmitter, for example, are well known in the art and need not be discussed in detail here. In fact, depending upon the use of the ASIC, various functions may be provided on the chip. In addition, thereto, the integrated circuit is provided with a unique serial number. In this scheme, 22 serial number bits are provided. When the ASIC is in the wafer stage of its manufacturing cycle, the chips are tested and malfunctioning chips are marked for eventual deletion by automatic test equipment. The remaining chips which are acceptable are then made to undergo an additional operation in accordance with this scheme. The 22 serial number bits are uniquely configured in a sequential manner for each "good" chip starting with the serial number 0 to a number corresponding to the actual number of chips to be fabricated for a given order. For example, the first 10,000 chips will be numbered sequentially from 0-9,999. Subsequent manufacturing runs of this chip will start from the next available number and continue for the number of chips required in that manufacturing run. The utilization of 22 bits yields 2.sup.22 or 4.2 million unique combinations. This provides a very low probability of repeating numbers occurring in the same general area within the physical range of a wireless system or within the same wired system. An advantage of this scheme is that the user need not enter a multi-digit code in order to activate the security system either to arm or disarm the system or to perform another function. The code is transmitted automatically. A further advantage is that each part has a unique number, which has been preprogrammed at the factory. Thus, neither the user nor the installer need know the unique number. The microprocessor control controller 334 is simply informed that a particular user, for example assigned as user 1, is to be associated with a unique serial number which is then entered into the computer via activation of the ASIC. Once the identification number , for example 1, and the parameters are entered into the computer, the sensor can be activated to trigger the ASIC to transmit its unique serial number to the computer, which will then "memorize" the unique serial number associated with that newly programmed sensor. Again, this can be accomplished without the installer having knowledge of the unique serial number and avoids the difficulties in programming such numbers found in the prior art. Thus increases in the number of bits in the address and increases in the number of devices on a single polling loop exacerbate the problem of the need to poll each device via a unit poll within a short enough cycle so that the system can operate in real time.